1. Field of the Invention
This invention relates to microelectronics, including vacuum microelectronics, in particular to filed emission devices, specifically to filed emission cathodes, as well as to other field emission devices such as field emission displays, electron sources for electron guns, for microwave devices, etc.
2. Description of the Related Technology
During the last few years, various versions for realization of field emission, including the emission with using of defects in planar structures, have been considered, the defects acting as initiators of the field emission [1,2]. Field emitters such as tips and blades prepared by special methods, as field emission initiators, have many advantages in comparison with the defects from the point of view of feasibility to realize regular multiple arrays of the field emitters and controlled growing of the arrays on large areas. However, cases often occur at the practice when the regular arrays are inferior to structures with an incidental distribution of the defects in homogeneity.
Troubles in stability and controllability of electron flows given off by the field emitters are also known. Troubles with uniformity of the field electron emission of the multiple field emitter arrays are of the same nature. The uniformity is typically ensured by ballast resistors that equalize electron currents through different field emitters of the multiple field emitter arrays.
Various design and technological solutions are used for overcoming of the troubles (problems) with the field emitter.
A controlled electron source is known where the field emitter is connected to the drain of MOSFET that serves as a stable current electron source [3,4]. In such an electron source, the issue of stability and controllability of the field emission current is successfully solved. However, transistor p−n junctions in the electron source are placed in the substrate where the field emitter is placed, too, and a substrate, too. This increases significantly the area taken by a pixel and, accordingly, decreases the resolving power of field emission displays based on such electron sources.
A solution of the problems of stability and controllability combined with the spatial arrangement of the control components is successfully realized in the patent [5]. FIG. 1 illustrates the field emission cathode according to the prior art [5]. In FIG. 1, reference numerals 1-4 represent a substrate, a cathode, a diode, and a metallic layer, respectively. Also, reference numerals 5-8 represent a semiconductor layer, an emitter, an insulating layer, and a control electrode, respectively. Here, in the electron source the diode (3) is placed in the emitter base for the stability and the controllability of the field emission current. Such a design decreases principally the sizes of the electron source three time, as minimum, because its control component takes the same place as the field emitter itself. Such an electron source allows to regulate the voltage so that the starting voltage for the field emission is decreased and, in such a way, the uniform emission is ensured. A plurality of emitters, acting through diodes and operating actually as ballast resistors, are placed onto the cathode electrode. Such a design ensures the uniformity of the field emission and, simultaneously, its controllability. However, the proposed in [5] components of stabilization and control of the field emission current are insufficient for successful solving of the problems of uniformity and controllability.
In the patent [6] a more complete using of the advantages of the field emitters is realized. The field emitter is considered as a spatially distributed object (various parts of which serve as functional components of a device) rather than as a “material point” of the field emission, without spatial characteristics of their various parts.
According to the patent [6] components for control of the electron source are transformed from the planar arrangements, as it was done in [3,4], into a vertical arrangement. FIGS. 2a and 2b illustrate the field emission devices according to the prior art [3]. Thus, a principal role in the stabilization and control of the field emission current is assigned (allocated), to the body and to the surface of the field emitter, in addition to the usual role of its top.
FIGS. 3a and 3b illustrate the field emission devices according, to the prior art [6]. In FIGS. 3a and 3b, reference numeral 01 represents a top of field emitter. Reference numeral 02 represents a control electrode. Reference numerals 03 and 03a represent an insulator. Reference numerals 04 and 06 represent a barrier (junction). Reference numeral 08 represents a control electrode. Reference numerals 09 and 09i represent a conductive part of substrate and an insulator part of substrate, respectively.
FIG. 3c illustrates the field emitter with various function areas of the prior art. In FIG. 3c, reference character E represents an external electric field. Reference character Ej represents various positions of a junction boundary (for example, p−n junction) under the influence of external electric fields of various values. Reference character Et represents the position of the junction boundary when electrons start to flow through the junction Reference characters l and d represent the length of the active area and width of the active area, respectively.
FIG. 3d illustrates the method for preparation of the field emitter according to [6]. Reference numerals 12, 13, and 14 represent layers with different kinds of conductivity. In FIGS. 3a-3d, reference characters a, b and c (i.e., not associated with any reference numeral) represent areas of various kinds of conductivity. Reference character e represent position of active areas. Similar to [3-5] in the patent [6] an extracting electrode acts to electrons placed in the emitter top. In [6] electron sources are considered where the field emitters have sufficient length and thickness. Therefore, from the point of the action of the control electrodes or barriers (such as the diode in [5]), as minimum four areas of the electron sources are considered:—the substrate on which the field emitter is placed;—basis of the field emitters;—top of the field emitters;—their bodies. These are areas of selective activation, or active areas. So, the active area is an area in the substrate, in body of the field emitter, in its basis or at its top. A connection of the source of the charge carriers with the field emitter is implemented through the areas, and a control of the field emission current (of the charge carriers flow) from one area to another by means of stimulation and extracting is implemented.
In some cases, however, such a control of the charge carrier flow can not be realized in [6]. This is related to the fact that the field emitter, being under the action of a rather high electric field, for example, of the anode one, is subjected to its influence not only to the area of the top of field emitter but also all over the body. As a result, such an electric field, acting to the field emitter, “shorts out” an action various barriers and over control components. The method for preparation of the field emitters by “wet” or “dry” etching used in the patent [6] results in formation of the emitters having small ratios of the length l of the active area to its diameter d. In this case, for controlling of the field emission current, a very large voltage must be used in order to compensate the action of the large external (for example, of anode) electric field.
Indeed, if the field emitter, containing a part with the p-type conductivity is placed in the electric field E (FIG. 3c), formed by the anode, the boundary of the first of the first p−n junction 04 is shifted Ej to the p-area. At a certain value Et, the first junction 04 approaches to the second one 06 in such an extent that the electrons from the n-area c begin tunneling through the narrowed barrier to the field emitter. This causes emission of electrons from field emitter. This is the “shorting out” under the external electric field. Existence of the control electrode near the field emitter both in traditional (FIG. 3a) and in the considered [6] version (FIG. 3b) can compensate the action of the penetrating electric field and, such a manner, to “lock” the charge carriers of the second n-area c. However, it is known that, at the geometric sizes, considered in [6], the length l of the p-area is compatible with or and even shorter than the width d. As it is known, for “locking” of the charge carriers value of the traverse electric field of the control electrode 02 or 08 must be comparable with the longitudinal field responsible for the charge carrier flow. This makes it necessary to apply large voltages to the control electrodes.
In addition, in the patent [6] the control electrodes stimulate the flowing of the charge carriers through the active area and extract the electrons from the field emitter. In such a way, the electron emission is stabilized and controlled. At the same time the control electrodes in [6] does not lock the flow of the charge carriers through the active area. The above function of the control electrodes-to stimulate the flowing of the charge carriers, makes it necessary mentioned in [6] approximate sizes of p-area as“ . . . formed to no more than several microns in thickness and generally to submicron order thickness” (see column 8, last paragraph in [6]). This means that the authors of [6] did not consider a possibility to provide the control electrode by “locking” function and, as a result, they considered the design which is enough just for stimulation and which; is not enough for locking the electrons moves under the influence of strong external electric field. However, it is known that if the control electrodes can lock the flow, it is possible to use small (in absolute value) negative voltage for the locking of the flow. This approach is very important from practical point of view-to use low voltage “electric keys” in different driving systems, for example, in the field emission displays. Such a version can not be realized in [6] due to small value of the characteristic l/d that is there approximately equal to 1 which is provided by the design proposed in [6].